Engine Control Module: Description and Operation

DME control module signal output
If a fault is detected and stored, the prevailing ambient conditions are also stored and can be read out later with the generic scan tool. This makes troubleshooting much easier because operating data are available that were present at the moment the fault occurred.

These ambient conditions of a fault are stored the first time the fault occurs. If the same fault occurs a second time, these ambient conditions are also stored. Each further occurrence of a particular malfunction updates the ambient conditions of the last time the malfunction occurred, i.e. the ambient conditions present the first and last time the same malfunction occurred can be read out.

These ambient conditions can be read out only with the Porsche System Tester 2.

The ECM must be able to give out the following signals (freeze frame data) with a generic scan tool (SAE J1979):

- Diagnostic Trouble Codes
- Engine load
- Engine coolant temperature
- Oxygen sensing status (oxygen sensor)
- Oxygen sensing adaptation values
- Fuel pressure (when available)
- Ignition angle
- Intake air temperature
- Intake manifold pressure (when available)
- Air mass value
- Engine speed
- Throttle position
- Secondary air status
- Vehicle speed

Standardised DTC to SAE J 2012 (DTC-Diagnostic Trouble Codes)

Diagnostic Trouble Codes that can be read out by the ECM are standardised, which means that all manufacturers must use the same Diagnostic Trouble Codes.

The DTC is always a 5-character alphanumerical value, e.g. "P0100".

All P0xxx codes are standardised. However, each manufacturer may use additional codes which go beyond the standardised ones. This is the case if the manufacturer has integrated additional functions in the ECM beyond those required by law, and these can also be diagnosed. These codes are referred to as P1xxx codes, e.g. "P1100".

The first character (letter) of a code identifies the system which has input the code. A total of four system types are covered:

- B for body
- C for drive train
- P for drive
- U for future systems.

The OBD II only requires the P code.

The second character represents the standard code (P0xxx) or manufacturer code (P1xxx).

The third character indicates the system assembly from which a fault occurred.

These are:
P01xx Fuel
or and
P02xx air metering
P03xx Ignition system
P04xx Additional emission control systems
P05xx Speed and idle air control
P06xx Computer and output signals
P07xx Transmission

The fourth and fifth characters represent the components or systems.